Expression of prostaglandin E 2 and oxytocin receptors in stallion accessory sex glands

It is unknown whether oxytocin and prostaglandin receptors are present in accessory sex glands. Furthermore, if receptors are present, their importance and distribution is unclear. Knowledge of normal receptor expression in intact and castrated males may help to improve semen collections in difficult stallions and may further our understanding of accessory sex gland physiology. Objectives were to characterize expression of prostaglandin E 2 (EP2, EP4) and oxytocin (OXTR) receptors’ genes in equine accessory sex glands using immunohistochemistry (IHC) and to determine if localization varied based on age or reproductive status. We hypothesized that EP2, EP4, and OXTR are more strongly expressed in mature stallion glands than in gelding or fetal glands. At euthanasia, ampulla, prostate, vesicular, and bulbourethral gland tissue were collected from mature stallions (> 5 years old, n = 3), mature geldings (> 5 years old, n = 3), and male fetuses (280 days of pregnancy, n = 3). Fresh tissues were fixed in 10% neutral buffered formalin, then embedded in paraffin until processing. Tissues were sectioned in 5 µm sections and stained with rabbit antihuman polyclonal antibody for EP2 or EP4 (Santa Cruz Biotechnology), or mouse antihuman monoclonal antibody for OXTR. Slides were processed using an IHC Select HRP/DAB kit (Millipore, Burlington, MA). Protein localization of EP2, EP4, and OXTR was evaluated by IHC, and staining was characterized as absent, mild, moderate, or strong. Prostaglandin E2  and oxytocin receptors’ genes were expressed in all glands. EP2 expression was mild to moderate in luminal epithelium of all glands. EP4 was strongly expressed in luminal epithelium of all glands, moderately expressed in smooth muscle of ampulla and prostate, and mildly expressed in submucosa of vesicular gland. Presence of EP2 in lumen should not come as a surprise, since PGE2 /EP2 play an important role in secretory epithelium in other organs (e.g. lungs and stomach). Interestingly, upregulation of EP2/EP4 are associated with tumor development in accessory glands of humans and rodents and other tissues; however, neoplasia in accessory sex glands of stallions is extremely rare. EP2 and EP4 play an important role in contraction and relaxation in the esophagus and it is likely they play a similar role in the stallion accessory sex glands. Moderate expression of OXTR in epithelium and mild expression in glandular stroma, and its receptor presence suggested that oxytocin may help regulate glandular secretions. Presence in equine fetus suggested that these receptors may play a role in development of accessory sex glands. Lack of apparent variation in these receptors in castrated horses suggested a steroid-independent role for receptor expression in glands. In conclusion, expression of EP2, EP4, and OXTR was confirmed in all male equine accessory sex glands, including equine fetus. Relative roles of these receptors in stallion accessory sex glands physiology, during ejaculation, and disease remain to be studied.

Keywords: Prostate, horse, ampulla, seminal vesicle

This manuscript was originally published in the journal Clinical Theriogenology Vol 12(3) Sept 2020.  Clinical Theriogenology is the official journal of the Society for Theriogenology (SFT) and the American College of Theriogenologists (ACT).  This content has been reproduced on the IVIS website with the explicit permission of the SFT/ACT.